Field of the Invention
The present invention relates to an electrolyte comprising a eutectic mixture and a nitrile compound, and an electrochemical device containing the same.
Description of the Related Art
Various kinds of electrolytes are used for contemporary electrochemical devices, for example, lithium secondary batteries, electrolytic condensers, electric double-layer capacitors, electrochromic display devices, and dye-sensitized solar cells as well that are currently studied in various ways for future commercialization, and the importance on the electrolytes is increased day by day.
In particular, lithium secondary batteries have high energy density and long life cycle, and thus they are the center of attention. Generally, a lithium secondary battery includes an anode made of a carbon material or a lithium metal alloy, a cathode made of lithium metal oxide, and an electrolyte made by dissolving a lithium salt in an organic solvent. Structural stability and capacity of lithium metal oxide are determined by intercalation and disintercalation reactions of lithium ions. The capacity of lithium metal oxide increases as a charge potential is raised, but the lithium metal oxide becomes structurally unstable accordingly. Such unstable structure of the electrode provokes the generation of oxygen, which may cause overheating in a battery or a reaction with the electrolyte, possibly resulting in explosion of the battery.
Currently, an organic solvent used widely for an electrolyte of a lithium secondary battery includes ethylene carbonate, propylene carbonate, dimethoxy ethane, gamma-butyrolactone (GBL), N,N-dimethyl formamide, tetrahydrofurane, acetonitrile, and so on. These organic solvents generally have high volatility and high ignitability, so a lithium secondary battery using such organic solvents may have a problem in its stability, particularly high temperature stability.
In order to solve this problem, it has been suggested to use an imidazolium-based or ammonium-based ionic liquid as an electrolyte of a lithium secondary battery. However, such an ionic liquid may be reduced in an anode at a higher voltage than lithium ions, or imidazolium or ammonium cations may be inserted into the anode together with lithium ions, which rather deteriorates the battery performance.
Meanwhile, Korean Patent Registration No. 10-751203 and Korean Laid-open Patent Publication No. 10-2007-85575 disclose, as an electrolyte, a eutectic mixture of lithium salt and an amide compound such as acetamide, urea, methylurea, caprolactam, valerolactam, trifluoroacetamide, carbamate, formamide, and so on, which is represented by certain chemical formulas. Such a eutectic mixture exhibits high thermal and chemical stability as well as relatively wide electrochemical window, and thus it solves the problems such as evaporation or ignition of an electrolyte caused by use of the existing organic solvents.
Accordingly, the development of various eutectic mixtures as an electrolyte is accelerated. In particular, there is an increased demand for an electrolyte capable of improving the high temperature stability of an electrochemical device.